1. Field of the Invention
The present invention relates to electro-optic field modulators and in particular to using a Mach-Zehnder modulator for generating desired encoding or modulating formats to optimize the tunable duty cycle RZ pulses for optimizing system performance.
2. Technical Background
Current optical communication systems are characterized by high bit rates and complex channel and network architecture. Owing to the variety of power levels, dispersion maps, and system reach, the range and magnitude of system impairments due to both linear phenomena (such as dispersion and ASE) and nonlinear phenomena (such as self-phase modulation SPM, cross-phase modulation XPM, four-wave mixing FWM) can be quite significant. The choice of a modulation format has a significant impact on the interplay between the fiber and system parameters, particularly in the next generation of optical communication systems that will employ dense wavelength-division multiplexing (DWDM) and/or high bit rates of 40 Gb/s or even higher. For example, if the format is not optimized for a particular system, the optical pulses may be distorted such that the output optical signal is significantly degraded due to the appearance of side lobes on the edges of the pulses.
Nonlinear mixing of closely spaced pulse pairs generates undesired temporal side pulses or ghost pulses due to time domain four-wave mixing. The ghost pulses fall on the center of adjacent bit slots. As a result, the ghost pulses falling on the bit xe2x80x981xe2x80x99 or Mark pulses, cause amplitude jitter which is one of the dominant penalties at the high bit rates, such as 40 Gb/s or greater.
Generating return to zero (RZ) pulses with standard RZ or carrier suppressed RZ (CSRZ) arc known. Standard modulation formats include non-return-to-zero (NRZ and return-to-zero (RZ) with 50% and 33% duty cycles. In general, the RZ modulation format has better performance than NRZ format in high-data-rate ultra-long-haul transmission systems and more robustness to nonlinearity impairments. This advantage, however, depends on several factors, including the appropriate balance of the launched power, the RZ pulse width (duty cycle), the optical and electrical filter bandwidths and the dispersion map. Variants of the RZ modulation format have also been proposed, including chirped-RZ (CRZ), carrier-suppressed RZ (CSRZ), differential phase shift keyed (DPSK), and duobinary.
Many have tried to optimize system performance. Variable duty cycle RZ pulses have been produced using an optical delay interferometer and are predicted to improve the bit error rate by about 2 dB. A reduction in the electrical driving voltage of a duobinary transmitter has been found in another approach to enhance the extinction ratio and improve the performance in standard single mode fiber. Even though the duty cycle was not discussed in yet another approach, the simulated optical spectrum shows a reduction in the intensity of the side lobes when the electrical driving voltage is reduced to 25%. However, there is no discussion of simultaneously tuning the bias voltage to optimize the shape of the duobinary pulses in this approach. It has been shown in a WDM experiment that optimizing pulse width of RZ pulses can suppress XPM penalties. Such various methods for tuning of the duty cycle show the benefit of duty cycle optimization.
Accordingly, there is a need to vary the duty cycle of different RZ modulation formats, in an improved manner, without increasing sidelobe intensities or other distortions to the optical output pulse.